Method for operating a combustion system and combustion system

ABSTRACT

A combustion system with a burner and a membrane unit including a retentate side and permeate side for separating oxygen from air is provided. The permeate side of the membrane unit is connected to the burner via a line. A heat exchanger is connected in a combustion gas flow such that a hot combustion gas arising from fossil combustion is applied to the primary side of the heat exchanger and air applied to the secondary side of the heat exchanger is heated to a temperature required for operating the membrane unit and fed to the membrane unit. Further, a method for operating the combustion system is provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of International Application No. PCT/EP2008/060152 filed Aug. 1, 2008, and claims the benefit thereof. The International Application claims the benefits of European Application No. 07015542.9 EP filed Aug. 7, 2007. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for the operation of a combustion system and a combustion system.

BACKGROUND OF INVENTION

During combustion of a fossil energy source, for example for the generation of steam in power plants, the combustion gas (flue gas) thereby arising is loaded inter alia with CO2, which is regarded as an alleged main cause of the climate change-inducing greenhouse effect.

With the aim of permanent storage (sequestration) in geological formations such as oil or natural gas deposits, aquifers or coal seams, CO2 can be separated out with different methods in the power plant process.

CO2 separation after combustion (post combustion capture) from the N2/CO2/H2O mixture of the combustion gas at the cold end of a conventional power plant process through absorption (chemical and physical), adsorption, liquefaction or by means of membrane methods is costly. The energy required for this is, for example, removed from the process steam and leads to considerable efficiency losses. The advantage of post-combustion CO2 capture nevertheless lies in the fact that the intervention in the power plant process is of a lesser extent compared with other methods and this method is thus also suitable for retrofitting to existing power plants.

In the case of a coal gasification method for the capture of CO2 prior to combustion (pre combustion capture) the fuel coal is initially gasified (integrated gasification combined cycle, IGCC power plant) and the CO here reformed with water vapor into H2 and CO2 (water gas reaction or “CO-Shift).

CO+H2O→CO2+H2

The CO2 can now be removed from the combustion gas in a higher concentration and under high pressure through physical absorption. The remaining hydrogen is available for the generation of electricity by means of gas and steam turbines. The exhaust gas which ultimately leaves the power plant is thus practically CO2-free.

In the case of the so-called oxyfuel method, oxygen is fed as the oxidation means to the combustion instead of air. The method has a number of advantages.

Firstly it is simpler to separate CO2 from the combustion gas highly enriched with CO2, than from the exhaust gas stream after the combustion with air, because the combustion gas essentially comprises CO2 and H2O. Secondly the combustion gas energy losses resulting from the absence of the ballast portion of the air-nitrogen in the combustion gas are reduced, and thirdly the nitrous oxide emissions caused by the air-nitrogen are avoided.

As combustion in pure oxygen would, however lead to excessively high combustion temperatures, the oxygen captured from the air is mixed with a recirculated portion of the combustion gas and only then fed to the combustion. The returned combustion gas thus replaces the air-nitrogen.

Air can be broken down into its component parts (that is in the first instance nitrogen, oxygen, as well as inert gases) by a variety of methods. The technically sophisticated, but energy-intensive, cryogenic air breakdown (low temperature air breakdown) breaks down the air into its major components nitrogen and oxygen in a mechanically driven thermodynamic process, and encompasses the substeps compression of air, removal of water vapor and carbon dioxide, fractional distillation (rectification) and refrigeration by means of throttling expansion.

Now, as an alternative to the cryogenic air separation systems, membrane-based units are proposed for the capture of oxygen (oxygen transport membrane OTM). The membrane effects—osmotic or electrochemical or also indexed by the partial pressure difference of the oxygen between the oxygen-emitting air side (retentate side) and the oxygen-absorbing side (permeate side)—lead to an at least partial capture of the oxygen from the air. Replacement of the cryogenic air separation system by a membrane is regarded as a possible option for enhancing efficiency levels in CO2-free power plant concepts.

Membrane-based arrangements for air breakdown are however disadvantageous in that the membrane unit must be kept at a comparatively high operating temperature in the 700° C. to 1000° C. range (high temperature-membrane method), so that the membranes can carry out their function. Accordingly heat energy must be permanently fed to the membrane reactor, so that the latter exhibits the necessary process temperature for oxygen capture from the air.

The oxyfuel concept, in which the membrane is a central element of the process, is for example known from the AZEP Project (Advanced Zero Emission Power Plant, 5^(th) Framework Program of the European Union, Project Number: ENK5-CT-2001-00514; CORDIS, Community Research and Development Information Service) or also from the OXYCOAL-AC method, further developed by RWTH Aachen and supported and co-financed by RWE Power AG, E. ONEnergie AG, Siemens AG, Linde AG and WS-Wärmeprozesstechnik GmbH.

The configuration of the combustion process of the OXYCOAL-AC method is outlined in FIG. 1. One significant feature of this process is the high temperature membrane system integrated into the plant layout for the capture of oxygen, which is supplied with air on the high pressure side and which gives off oxygen to the recirculated combustion gas stream via an ion-conducting membrane at operating temperatures of approximately 800° C. As a result of this operational management, a high partial pressure gradient and thus a high degree of permeability for the oxygen are achieved. The gas mixture fed from the high-temperature membrane system to the burners essentially comprises CO2, H2O and O2 and has approximately the same proportion of oxygen as the combustion air in the case of conventional combustion, so that the combustion temperatures customary in combustion technology should also be set. The separation of the water from the combustion gas removed from the process can take place by means of condensation.

SUMMARY OF INVENTION

This arrangement does, however, have the disadvantage that, if oxygen is generated at the periphery of the process, the combustion gas must be transported from the steam generator to the periphery and that to reduce the impact on the oxygen-absorbing side of the membrane from pollution of the combustion gas, a hot gas scrubbing process must be interposed.

It is an object of the invention to propose a method for the operation of a combustion system and a combustion system which overcome the abovementioned disadvantages in the case of membrane-based oxygen capture.

This object is achieved in accordance with the invention by the features of the independent claims.

Further advantageous embodiments are cited in the dependent claims.

In the inventive method heat energy is fed to the membrane in order to maintain the required process temperature, whereby the heat energy is obtained from the combustion gas produced during operation of a burner, in a heat exchange with the air, and the heated air is fed to the membrane, and the oxygen separated out is carried away from the membrane via a line.

The heat exchange process and its advantageous linkage with the high temperature level of the combustion gas produced during operation of the burner provides a particularly efficient method of heating up the air to the requisite process temperature, that is a temperature required for operation of the membrane unit, and then feeding the heated air to the membrane unit already at the correct temperature. The membrane can hereby be brought to the operating temperature, typically 700° C. to 1000° C., in a particularly simple manner.

The temperature of the oxygen separated out in the membrane is likewise essentially the original process temperature and can, before it is fed into the gas carrier for the combustion (usually CO2), advantageously be used to heat a part of the air which is to be fed to the membrane.

As the partial pressure of the oxygen is the driving force for oxygen capture by means of the membrane, it is expedient to convey the oxygen separated out of the air through the membrane away from the permeate side of the membrane.

After the heat exchange with the air, part of the combustion gas is preferably mixed with the oxygen specifically in such a way that the mixture has approximately the same proportion of oxygen as the combustion air in the case of conventional combustion, so that the combustion temperatures customary in combustion technology can be set, if the oxygen/combustion gas-mixture is fed to the burner for combustion with the fossil fuel.

The depleted air leaving the membrane unit on the retentate side, which is essentially still heated to the original temperature, that is the process temperature, is advantageously available for further useful applications, whether, for example, by employing the heated air by transferring the heat to a water vapor circuit or to drive an expander of a gas turbo generator, which advantageously is in turn used for the compression of the air to be fed to the membrane.

Compared with the OXYCOAL-AC method, this method has a number of advantages. Firstly, combustion gas does not come into contact with the membrane, for which reason no hot gas scrubbing is required. In the case of this configuration, the membrane exclusively comes into contact with air. The possibility of damage to the membrane material caused by the combustion products contained in the combustion gas is thus excluded.

Secondly, the hot combustion gas only needs to be transported in the comparatively short recirculation line.

Thirdly, a compact structure of the heat exchanger in the steam generator and a compact structure of the hot air lines is possible, as the hot air is usually present at pressures >15 bar.

In addition the membrane technology can be integrated into first-generation oxycoal methods.

The inventive combustion system, in particular for execution of the inventive method, comprises a burner and a membrane unit, comprising a membrane, a retentate side and a permeate side, for the capture of oxygen from air, where the membrane unit is connected to the burner with its permeate side via a line, where a heat exchanger is positioned in the combustion gas stream in such a way that on the primary side this can have hot combustion gas arising from the fossil combustion applied to it and on the secondary side the air that can be supplied to the heat exchanger can be heated to a temperature required for operation of the membrane unit and can be fed to the membrane unit, and the oxygen separated out in the membrane unit can be carried away from the membrane via a line.

An oxygen/air-heat exchanger is preferably positioned in the line between the permeate side and burner, in order to lower the temperature of the oxygen separated out in the membrane, which still essentially corresponds to the original process temperature, to such an extent that a recirculated combustion gas, that is CO2 stream, is not or only insignificantly increased in temperature through its mixing with oxygen. Advantageously, the oxygen/air-heat exchanger is positioned on the secondary side upstream of the retentate side of the membrane for feeding of the heated air.

Combining of the hot air from the heat exchange with the combustion gas and the heated air from the heat exchange with the separated-out oxygen prior to its application to the retentate side is expedient.

A fan in the line connected with the permeate side of the membrane is also advantageous in relation to the partial pressure gradient of oxygen via the membrane.

A recirculation line for combustion gas and the connection of this recirculation line with the oxygen-bearing line between the permeate side of the membrane and the burner is further advantageous. The return of the combustion gas (usually CO2) produced during operation of the burner, and cooled in a heat exchange with the air, and the feeding of the separated-out oxygen into this recirculated combustion gas prevent the high combustion temperatures as otherwise arise in the combustion of fossil fuels with pure oxygen.

The retentate side of the membrane is advantageously connected with a compressor, to which air can be fed for the breakdown with the membrane, and which is coupled with an expander, to which the oxygen-depleted air, which is still essentially heated to the original temperature, that is the process temperature, and leaves the membrane unit on the retentate side, can be applied.

The membrane is preferably an oxygen ion-conductive membrane.

The combustion system expediently comprises a fossil-fired steam generator.

The combustion system is particularly advantagously in a steam power plant with a steam turbine, in particular, if this steam power plant comprises a CO2 separator, by means of which the highly enriched CO2 can be separated out of the combustion gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail by way of example on the basis of the drawings.

FIG. 1 shows a circuit of the combustion process of the OXYCOAL-AC method, and

FIG. 2 shows a basic circuit diagram of the inventive combustion system, taking a coal dust-fired steam power plant as an example.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows the circuit of the combustion process of the OXYCOAL-AC method known from the prior art. The combustion system 1′ has a steam generator 2 with a burner 3. The steam generator 2 is positioned in the water vapor circulation of a steam turbine system which is not represented in greater detail.

The burner 3 has a feed line for the fossil fuel 4. A feed line 5 for an oxygen/combustion gas mixture is further present, which leads to the burner 3. In the steam generator 2 the fossil fuel is burned together with the oxygen/combustion gas mixture, whereby the water in the pipework system of the steam generator is converted to steam at a high temperature.

In the case of the OXYCOAL-AC method, oxygen 25 is separated out of air at a process temperature by means of a membrane 6 of a membrane unit 26, whose retentate side 7 is supplied with pressurized air 22, and given off to a stream of recirculated combustion gas 9. The heat energy required to maintain the necessary process temperature is obtained from the combustion gas 9, which is fed to the permeate side 8 of membrane 6 and carried away from it again with the oxygen 25 via a recirculation line 28 (sweep gas). As a result of this operational management, a high partial pressure gradient and thus a high degree of permeability for the oxygen 25 are achieved.

The oxygen/combustion gas mixture 10 essentially comprises CO2, H2O and O2, and has approximately the same proportion of oxygen as the combustion air in the case of conventional combustion, so that combustion temperatures customary in combustion technology can also be set. This mixture 10 is fed to the burner 3 for the reaction with the fossil fuel 4.

The air 24 at least partially depleted through the removal of oxygen 25 by means of the membrane 6 is fed to an expander 11. Coupled with the expander 11 on the same shaft 12 is a gas turbo generator, whose compressor 13 sucks in air 21 and applies compressed air 22 to the membrane 6 via a compressed air line 29.

To reduce the impact on the membrane 6 caused by pollution of the recirculated combustion gas 9, a hot gas scrubbing system 14 is interposed between the steam generator 2 and the membrane 6. A fan 15 supports the combustion gas circulation.

As an exemplary embodiment of the inventive combustion system 1, FIG. 2 shows the configuration plan representing the principle of a coal dust-fired steam power plant. The following descriptions are essentially limited to the differences relative to the exemplary embodiment of the prior art from FIG. 1, to which reference is made in relation to features and functions that remain the same. Components essentially remaining the same are in all cases identified using the same reference numbers.

In the exemplary embodiment in FIG. 2, in order to maintain the required process temperature of the membrane 6, heat energy is not fed directly via the combustion gas, but instead via the air 23, which is to be applied to the retentate side 7 of the membrane 6. This air is heated in a combustion gas/air-heat exchanger 17, in heat exchange with the combustion gas, to between 700° C. and 1000° C., preferably 800° C. to 900° C., in order to guarantee a sufficiently high operating temperature of the membrane 6.

The oxygen 25 accumulating at the permeate side 8 of the membrane (permeate) is carried away via line 27 with the aid of a fan 16 and fed to the burner 3.

A further heat exchanger 18, which is integrated into this oxygen path has the task of reducing the oxygen temperature from the membrane operating temperature as far as possible such that—in the case of dust-fired steam generators 2—the stream of carbon dioxide 9 returned via a recirculation line 28 is not, or is only insignificantly increased in temperature as a result of mixing with the oxygen 25. Part of the compressor discharge air 19 is here applied to the secondary side of this heat exchanger 18 via a compressed air line 29, which is thereby heated and mixed into the hot stream of air 20 coming from the steam generator 2.

By means of this operational management, no direct contact between the combustion gas 9 and the membrane 6 occurs. Neither is any hot gas scrubbing 14 thus necessary. 

1.-26. (canceled)
 27. A method of operating a combustion system with a burner operated with a fossil fuel, comprising: separating oxygen out of air by a membrane at a process temperature; feeding the separated-out oxygen to the burner for combustion with the fossil fuel, a hot combustion gas being formed; feeding heat energy to the membrane to maintain the required process temperature, the heat energy being obtained from hot combustion gas in a heat exchange with the air; feeding heated air to the membrane; and carrying away the separated-out oxygen away from the membrane via a line.
 28. The method as claimed in 27, wherein the air is heated in heat exchange with the combustion gas to 700° C. to 1000° C.
 29. The method as claimed in claim 27, further comprising: cooling down the separated-out oxygen.
 30. The method as claimed in 29, wherein the separated-out oxygen is cooled in a heat exchange with air, the air being warmed.
 31. The method as claimed in claim 30, wherein the warmed air is mixed with the air heated through the heat exchange with the combustion gas, and the mixed heated air is fed to the membrane.
 32. The method as claimed in claim 27, further comprising: mixing a part of the combustion gas with the oxygen after the heat exchange with the air.
 33. The method as claimed in 32, wherein the combustion gas-oxygen mixture is fed to the burner for combustion with the fossil fuel.
 34. The method as claimed in claim 27, further comprising: feeding the air which is at least partially depleted through the removal of oxygen by the membrane to an expander.
 35. The method as claimed in claim 34, wherein the expander drives a compressor, the compressor applying compressed air to the membrane on a permeate side.
 36. The method as claimed in claim 27, wherein 80 to 100% of the air is fed to a heat exchange with the hot combustion gas, and wherein 0 to 20% of the air is fed to a heat exchange with the oxygen separated out of the air.
 37. A combustion system, comprising: a burner; a line; a membrane unit including a membrane, a retentate side and a permeate side, for capturing oxygen from air, the membrane unit being connected at the permeate side to the burner via the line; and a heat exchanger positioned in a stream of combustion gas, wherein a hot combustion gas produced from fossil firing is applied to the heat exchanger on a primary side, the air supplied to the heat exchanger is heated to a temperature required for operating the membrane unit on a secondary side and is fed to the membrane unit, and the oxygen separated out in the membrane unit is carried away from the membrane via the line.
 38. The combustion system as claimed in claim 37, further comprising: an oxygen/air heat exchanger arranged in the line between the permeate side and the burner, the oxygen/air heat exchanger being positioned on the secondary side of the retentate side.
 39. The combustion system as claimed in claim 37, wherein the air heated to a process temperature in the heat exchanger is mixed with air warmed in the oxygen/air-heat exchanger on the secondary side and is fed to the retentate side.
 40. The combustion system as claimed in claim 37, further comprising: a fan connected in the line between the permeate side and the burner.
 41. The combustion system as claimed in claim 37, further comprising: a recirculation line for the combustion gas, the recirculation line being connected to the line via which the permeate side is connected to the burner.
 42. The combustion system as claimed in claim 37, further comprising: an expander; and a compressor, wherein the retentate side is connected with the expander for use of thermal and mechanical energy of a retentate, and wherein the expander is coupled to the compressor.
 43. The combustion system as claimed in claim 42, wherein the compressor is an air compressor to which the air to be separated by the membrane is fed, the air compressor being connected with the retentate side via a further line.
 44. The combustion system as claimed in 37, wherein the membrane is an oxygen ion-conductive membrane.
 45. The combustion system as claimed in claim 37, further comprising: a fossil-fired steam generator fired by the burner.
 46. A steam power plant, comprising: a steam turbine; a CO2 separator for separating CO2 out of a combustion gas; and a combustion system, the combustion system comprising: a burner; a line; a membrane unit including a membrane, a retentate side and a permeate side, for capturing oxygen from air, the membrane unit being connected at the permeate side to the burner via the line; and a heat exchanger positioned in a stream of combustion gas, wherein a hot combustion gas produced from fossil firing is applied to the heat exchanger on a primary side, the air supplied to the heat exchanger is heated to a temperature required for operating the membrane unit on a secondary side and is fed to the membrane unit, and the oxygen separated out in the membrane unit is carried away from the membrane via the line. 